You might want to build a "Formula builder" part to your project are as an activeX control, so you can control the flow of what is entered into the formula.
You can't far as I am aware take text from anything and insert into a variable for it to do computations on it. You can do some heavy string manipulation to decipher their formula and then control the way it is processed, but this is very difficult and time consuming. What are you trying to let them build formulas for? Is it a math program?